Polo-like kinase 1 (PLK1) plays a pivotal role at multiple steps of mitosis. Recently, PLK1 was implicated as an oncogene and has emerged as a potential target for anti-cancer drugs. Although significant progress has been made towards the understanding of the roles of PLK1 in cell cycle control, the molecular mechanisms by which it executes its functions during some important mitotic events remains elusive, largely due to the lack of identified physiological substrates for PLK1. This applicant presents preliminary data that focuses on a novel substrate for Xenopus polo- like kinase 1 (PLX1). The substrate has been identified and characterized as a deubiquitinase, whose mammalian homologue specifically modifies histone and affects cell division. The long- term goal of this proposed research is to define a novel mechanism by which PLX1 regulates the progression of cell cycle through the phosphorylation of this deubiquitinase. For the purposes of this application, the immediate or short-term goal is to clarify PLX1 regulation of the novel substrate and to determine the impact of such phosphorylation on the deubiquitinase. Since cyclin B/Cdk1 (MPF) is also suggested as a phosphorylating kinase for the deubiquitinase, our hypothesis is that PLX1 regulates the activity and possibly the localization of this deubiquitinase in vivo in coordination with MPF, which in turn, affects the progression of cell cycle. The hypothesis will be tested through the activities associated with following objectives: (1) Identify the main sites in the deubiquitinase phosphorylated by PLX1 and MPF; (2) Determine the impact of PLX1 phosphorylation on activity and localization of this substrate. The possibility that PLX1 phosphorylation activates the deubiquitinase and changes its localization will be examined; and (3) Determine whether phosphorylation of the substrate by PLX1 requires MPF priming. In many cases, PLX1 phosphorylation requires the priming of a PLX1 substrate by a distinct kinase in order to generate a binding site for polo box domain, a unique structure in the polo-like kinase family. The proposed studies are expected to result in accumulation of further preliminary data and lead to the submission of a full research proposal by the end of the project. PUBLIC HEALTH RELEVANCE: Polo-like kinase 1 is a key cell cycle regulator and has recently emerged as a promising target for anti-cancer drugs. This application proposes to study the regulation of a novel substrate for polo-like kinase 1. The study is expected to reveal a new mechanism of cell cycle control and may also provide a foundation for developing innovative strategies for treating human diseases such as cancer.